Plant species diversity indicators for use in the computation of critical loads and dynamic risk assessments

Soil models can be used to derive critical loads by computing the deposition that leads to critical limits for abiotic conditions, i.e. conditions that are just tolerated by an ecosystem. In this chapter various approaches are discussed to assess these critical limits for plant communities and plant species. Species diversity indicators have an important role in many of these approaches and such indicators may be based on species numbers, intrinsic values of species or the desirability of certain species or communities to be present in certain locations. Such desired or ‘target’ species or communities are often derived from concepts regarding the ‘pristine’ or ‘natural’ state of an ecosystem. For a diversity indicator the similarity of the actual or modelled state and the ‘target’ state has to be quantified, and various methods for quantification are discussed. Finally, a step-by-step approach is discussed to arrive at critical limits using niche models, based on various concepts regarding plant species diversity.